Vance Jaqua wrote:

Paul
A couple of curves to show propeller efficiency as
limited by the laws of physics. Since we are dreadful
short of sky hooks and tracter beams, the only
practical way to support and propell heavier than air
flying machines is to throw mass around for momentum.
And of course the handiest mass around is the air. If
we assume a cylinder of air the diameter of our prop,
we can use horsepower to accelerate this air to make
thrust, The power in the air is MV^2/2 (energy in foot
pounds) The thrust from this M delta V. We can get 100
units of force by either accelerating 1 unit of mass
100 ft/sec or 100 units of mass 1 ft/sec. Since the
power is a function of the velocity squared, we can
get
the force for the least power if we use lots of mass
and only speed it up a little bit. This is the why of
big props and large diameter helicopter blades.  Now
as we go faster, it is easier to get bigger mass,
because we are running into it - this is why smaller
props have less of a penalty than you might expect
when we use them on fast planes. For any given
diameter stream, and any given forward velocity, you
can compute the maximum possible thrust that you can
get,when you perfectly put power into accellerating
that stream (100 percent pumping efficiency prop). The
attached curves show this effect as a function of
horsepower per square foot of disc area, at various
speeds. One is in the form of maximum propulsion
efficiency you can get, and the other is maximum
thrust per horsepower you could get if your prop was a
perfect pump. The rest of the losses that get you down
to real propellers are primarily pumping losses -from
such diverse sources as airfoil drag, tip vortices,
turbulance etc. I did not make this up - these are Mr
Newton's laws - Vance

Thanks for the curves. I think we can  narrow the range from 100 to 1
down to 10 to 1 so this is less than  a primary effect.
I can see how this might affect a large turbo fan or helicoptor greatly.

Paul Lamar
 
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